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When you first run an Ethereum client, It will first need to sync with the current blockchain. Depending on your client, system, bandwith, this action can take very long, up to a few days. Please visit this ethereum stackexchange post for tips.
Visit the ethstats website to check last block number, and compare it with the block you are syncing on your machine. Once they are the same, you know your client is synced.
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